DEVKLOPMENT OF THE HEART IX MARSUPIALS. 497 



nervous .system in these stages. The brain and spinal cord, being 

 the first organs to attain any considerable degree of development, 

 are naturally the first to receive a vascular supply, and both ven.e 

 capitis lateralis and medialis persist for some time, forming a rich 

 supply of capillaries to the brain and surrounding the developing 

 cranial nerves. 



Final Summary and Conclusions. 



Tlie facts revealed by the study of early stages in the 

 development of Marsupials point to the conclusion that while 

 the initiation of head-fold formation is in all probability due to 

 the forward growth of the brain-plate, there occurs also an active 

 backward growth of the anterior intestinal portal. Thi.s process 

 is associated with the rapid expansion of the peiicardiuui which 

 occurs at this period of development, and which brings about the 

 backward and inward growth of the layer of splanchnopleure 

 limiting the pericardium. 



In the course of this inward closure, the pericardial cavity 

 extends to the ventro-latei'al and finally to the ventral side of the 

 lateral primordia of the heart, so that when the lateral portions of 

 the pericardium become incorporated in its median limb, the 

 heart primordia lie in the dorsal wall of the pericardium. 



The approximation of the heart-tubes after gut-closure is 

 due to the fact that, at this period, the pericardium grows 

 rapidly in length and decreases in width so that the heart-tubes 

 are brought together by longitudinal stretching of the pericardial 

 wall lying between them. 



Curvature of the heart is due to its rapid growth at a period of 

 less active extension of the pericardium. 



The first two aoi"tic arches in Perameles ai-e typical, and the 

 development of the veins of the head resembles that process in 

 other mammals in that the anterior cardinal vein is derived from 

 persistent portions of two primitive head-veins, the venfe capitis 

 medialis and lateralis. The posterior continuation of the vena 

 capitis medialis also contributes to the formation of the posterior 

 cardinal vein and is itself derived from the dorsal aorta. 



E.BFERBNOES TO LITERATURE. 



1. Bremer, J. L. — "The Development of the Aorta and Aortic 



Arches in Rabbits." American Journal of Anatomy, 



vol. XXX. 



2. Evans, H. M. — " On the Development of the Aortee, Cardinal 



and Umbilical Veins and other blood-vessels of Vei-tebrate 

 Embryos from Capillaries." Anatomical Record, vol. iii. 



3. Evans, H. M. — Development of the Vascular System, in 



Keibel and Mall's ' Text-book of Human Embryology.' 



